This invention relates to LCD projectors, and more particularly, to a method and apparatus for providing an LCD projector that eliminates inefficiencies present in prior systems by maximizing the useful area for projection.
Generic LCD projectors are becoming more widely used in business applications. These types of projectors are typically used for business presentations, educational sessions, etc.
FIG. 1 shows a typical reflective liquid crystal device (LCD) projector. The arrangement of FIG. 1 represents a monochrome type of projector, but extension to color systems is known to those of skill in the art. For example, color systems may be implemented using an X-cube and a plurality of different color specific modulators. This technique is well known in the art, and will not be described in detail.
In the arrangement of FIG. 1, LCD 101 is a matrix of reflective LCD elements. Each element may rotate the polarization of incident light by up to 90 degrees. A polarizing beam splitter 102 passes light polarized in a first direction but reflects light polarized in a second and orthogonal direction.
In operation, light from lamp 104 is prepolarized by prepolarizer 105 and transmitted horizontally through polarized beam splitter (PBS) 102. The light exiting lamp 104 is collimated into substantially parallel columns. The polarized light passes plane 107 and is incident upon LCD 101.
The elements of R-LCD 101 are arranged to change the polarization of the incident light in accordance with a video signal driving R-LCD 101. This technique results in the light being reflected back from R-LCD 101 in a variety of different states. More specifically, some of the light is reflected back after having its polarization rotated, and other light remains polarized in the same direction as when it was initially incident upon R-LCD 101. Additionally, the light may have its polarization only partially rotated, providing for shades of gray. Each of the numerous elements in the LCD matrix may independently rotate the polarization of incident light by a different amount.
Upon being reflected back, the light which has not had its polarization changed passes back through plane 107 and is absorbed. Light which was incident upon R-LCD elements and which did have its polarization changed will not pass through plane 107, but will instead be reflected up through the post-polarizer 109 for projection as an image through projection lens 110. Light which has had its polarization changed by some degree will partially pass and result in gray shades rather than black and white. In short, the polarization may be rotated by any amount between zero and 90 degrees, with angles between these two extremes representing shades of gray.
Several problems exist with R-LCD projectors of the type shown in FIG. 1. One problem is that the rays are not strictly S-polarized or P-polarized as they hit the plane 107. This results in decreased contrast. For a more complete description of this problem and a proposed solution, see U.S. Pat. No. 5,453,859, issued Sep. 26, 1995.
Another problem associated with the systems of the type shown in FIG. 1 is stressed induced birefringence in the PBS 102. This phenomenon occurs because the PBS is warmed non-uniformly by the optical beam passing through it. The differential warming of the glass induces stress in the glass, which in turn induces birefringence in the glass. Prior attempts at solving the problem have been less than optimum.
A second prior art LCD projector design utilizes an off-axis LCD projector of the type shown in FIG. 2. A lamp 104 and prepolarizer 105 transmit polarized light to a reflective LCD 101. The polarization of the light is then either changed or not, or changed to some degree, by the state of the various LCD elements. As was the case for FIG. 1, the LCD elements are driven by a video signal, and thus, the polarization of the reflected light beam 203 has been modulated in accordance with the video signal. That reflected light beam is then transmitted through a post-polarizer 109 for projection via lens 110.
The basic problem with the arrangement of FIG. 2 is the inefficient use of the pupil of the projection lens, which is located approximately at plane xe2x80x9cB.xe2x80x9d More specifically, a large portion of the optical beam that would otherwise be captured by the projection lens is blocked by the path of the light from lamp 104. Thus, the usable pupil of the system is approximately one quarter of the full pupil of the projection lens, significantly reducing system efficiency. FIG. 3 depicts the pupil utilization in the prior art off-axis projector such as that shown in FIG. 2. It can be seen that about three quarters of the pupil area is wasted.
In view of the above there exists a need in the art for an improved reflective LCD projector which can efficiently utilize a larger pupil area and which eliminates the foregoing problems.
The above and other problems with the prior art are overcome in accordance with the present invention. A light source provides light to a modulator, which reflects back a modulated light signal that has been modulated in accordance with a video signal to be imaged. A mirror is interposed between the light source and the modulator. A portion of the light from the light source is blocked from reaching the modulator by the back of the mirror, but the modulated reflected light is focused entirely on the reflecting surface of the mirror and reflected through a projection lens by the mirror.
The mirror is positioned such that it only blocks a small portion of the incident light from the light source, thereby increasing efficiency. It may be placed directly in the path of incident light. In an additional embodiment, the mirror may be curved.
In a preferred embodiment, the modulator is a matrix of R-LCD elements.
Color may be obtained by utilizing an X-cube or similar device.